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The effect of various metal closure liners on the shelf life of white wine
Brian Gray, KWV, P.O. Box 528, Suider-Paarl, 7624
Introduction
The production of quality wine is most often attributed to the skill of the winemaker, who is responsible for ensuring the perfect marriage between grape quality, yeast strains and maturation techniques that end in a great product. Once the wine is bottled, however, changes to the organoleptic quality of wine may be greatly influenced by the selection of bottle closure.
There are many cases where bottle closures have failed to provide sufficient protection to the wine, usually through increased oxygen permeability leading to oxidation or by showing a propensity to impart off-flavours. In particular the incidence of trichloroanisole (TCA) in natural cork has driven the search for effective alternative closures.
A trial was proposed by KWV to investigate the effect that various metal closure liners, commonly referred to as 'screw caps', had on the organoleptic quality and chemical stability of a white wine over an 18-month period.
Alternative Closures
Screw caps have been making significant inroads into the wine bottling industry for a number of years. Although it may be argued that screw caps were initially perceived as closures for cheap wine, quite a number of cellars, both here in South Africa and internationally, have bottled premium vintages with this alternative to natural cork.
A recently published book by Tyson Stelzer, Taming the Screw, provides a detailed look into the history, design and use of screw caps for the bottling of wine. Within the book, some of the criteria given for a successful wine closure include that it should, ideally, have no interaction with the bottled product, should offer a reliable seal, be easily removable, and be available at a low cost. By developing a better understanding of the relative effects that various metal closure liners have on a wine, an educated selection may be made to protect the integrity of the wine.
Materials and Methods
The trial investigated a total of seven different liners using a Chenin Blanc/Chardonnay wine. For each individual liner type used, bottles were filled either with or without the use of nitrogen sparging on the filling lines, giving a total trial group of 14 treatments. A control group was also bottled at the same time, using a 45mm natural cork of super quality. The list of liners used is given, in no particular order, in Table 1. After bottling, the samples were stored at KWV in Paarl under stable, but unregulated conditions. Approximately every 2 months, samples were taken for organoleptic and chemical evaluations to determine if the liners affected the wine differently.
At every bi-monthly evaluation, the samples were tasted firstly as individual groups consisting of three glasses each (intra-liner evaluation). One of the glasses was the control sample, while the other two were the nitrogen and non-nitrogen samples of the same liner and the panel was required to indicate preference for one of these three samples. A second group of seven glasses was also evaluated (inter-liner evaluation), where one sample from each individual liner, filled without the use of nitrogen, was represented. This evaluation was done independently of the control sample to establish a relative ranking between the treatments.
The chemical analyses for this trial included total and free SO2, residual sugar, total and volatile acidity, pH, total polyphenols and gas chromatography analysis of a range of volatile compounds. Furthermore, torque measurements were also taken throughout the trial to determine the slip and break torque values for each type of liner.
Results and Discussion
Intra-liner evaluation
Over the 18-months of the trial, the individual preferences for the three glasses in the intra-liner evaluation were scored and a percentage selection was calculated. Table 2 shows the percentages as broken down against the samples of liners filled under nitrogen, not filled under nitrogen and the control sample. Most of the respondents showed a preference for the samples filled under nitrogen. The exceptions to this were found in liner 4, where a preference was shown for the control sample, and in liner 7, where no particular preference was found.
Inter-liner evaluation
During the inter-liner evaluation, the panellists were asked to rank the seven liner samples and the results thereof are illustrated in Figure 1. The 3-dimensional histogram represents the rankings, showing each liner type in a different colour, with the total number of selections for each liner in a particular ranking position. It is evident from the figure that the Saran TIN and Poly/Alu liners, shown in light blue and red, had a higher number of selections in the top ranking positions than any of the other liners. Also, the EPE Saran liner, indicated in the dark purple, showed a higher selection in the third position, with some selection in the first position as well. By contrast, the Compound and PET/EPE 300 liners had a greater number of selections in the lower ranking positions.
TABLE 1: List of metal closure liners used in the trial
Liner Name Supplier Liner 1 Saranex Alfelder Meyer Liner 2 Saran TIN La Manufacture
Générale de Joints (MGJ) Liner 3 Poly-Alu Unipac Liner 4 EPE EVOH Tekni-Plex Liner 5 EPE Saran Tekni-Plex Liner 6 PET/EPE 300 Tekni-Plex Liner 7 30´23
Compound Metal Closures
Group (MCG)
TABLE 2: Percentage preference scoring for the intra-liner organoleptic evaluations
Liner 1 Liner 2 Liner 3 Liner 4 Liner 5 Liner 6 Liner 7 (Saranex) (Saran
TIN) (Poly-Alu) (EPE
EVOH) (EPE
Saran) (PET/EPE
300) (Compound) Sample A (with
N2) 46.5 44.1 41.9 20.9 51.2 55.8 37.2 Sample B
(without N2) 30.2 32.6 34.9 32.6 34.9 14.0 39.5 Control sample 25.6 23.3 27.9 41.9 18.6 30.2 27.9
TABLE 3: Ranking positions for the inter-liner evaluations over an 18-month period, arranged from most preferred to least preferred and indicating original liner numbers in parentheses
Ranking
Position Liner Type 1 Unipac
Poly/Alu (3) 2 Tekni-Plex EPE
Saran (5) 3 MGJ Saran TIN
(2) 4 Alfelder Meyer
Saranex (1) 5 Tekni-Plex EPE
EVOH (4) 6 MCG 30´23
Compound (7) 7 Tekni-Plex
PET/EPE 300 (6)
Combining the results into Table 3 gives an overall ranking for the trial, showing that Liners 2, 3 and 5 were the most preferred closures. Liner 4, the EPE EVOH, was almost equally selected in all of the ranking positions, indicating neither particular preference nor dislike for the closure. The Saranex closure (Liner 1) also showed an even distribution, but had more selections in the higher preference positions and was therefore ranked fourth. As was also indicated in Figure 1, the lower ranking positions were mostly comprised of the Compound and PET/EPE 300 closures.
Regarding the lower ranking of Liner 6 (PET/EPE 300), it is important to note that discolouration of the outer edges of this liner was observed after only three months of storage (Figure 2). No other liner showed similar discolouration, but it was not clear from the study whether it was the result of residual wine left inside the metal closure after filling, or of leakage during storage. It is possible that it indicated insufficient sealing capacity that could account for the poorer performance of the wine using this particular liner.
Torque measurements
The typical torque values that were observed throughout the trial are shown in Figure 3, and of particular interest are the high slip torques for liners 1, 4 and 5 with trial averages of 19, 21 and 18 inch.pounds, respectively. The other four liners had lower slip averages, ranging between 8 and 12 inch.pounds. All of these recorded values were within acceptable norms, but typically, values higher than 15 may prove difficult for some consumers to open, while values approaching 7 may indicate insufficient sealing capacity that could, in turn, imply oxygen ingress and/or the loss of volatile flavours. The lower values regularly recorded for liners 6 and 7 could be of concern when considering their relative performance in the tastings.
Figure 1: Histogram of inter-liner preference selection between liner types
Figure 2: Discolouration observed on the outer edges of Liner 6 (PET/EPE 300)
Chemical analyses
On the chemical analysis side, there were significant changes in total and free SO2 levels over time. An initial sharp decrease (ca. 5 ppm) was observed in the averages of both the total and free SO2 levels between the 1st and 3rd month after filling. Thereafter a more gradual decline was recorded, accompanied with an increase in standard deviation. By transposing the free SO2 data, the differences between the various treatments can be illustrated in Figure 4.
To clarify the relative performance between the liners, the bias imposed by the large initial decrease in SO2 was disregarded in Figure 4. Differences between nitrogen addition were observed particularly for the last two liners, where the A samples were filled under nitrogen and the B samples were not filled under nitrogen. Although these differences only became significant at the 10% level, similar patterns were not observed for the other liners, implying that the use of nitrogen sparging may be of greater importance when using one of these last two liners. In addition, Liners 2 and 3 maintained slightly higher free SO2 levels, lending support to their better performance in the organoleptic evaluation.
The gas chromatography analysis showed significant decreases in some of the acetate esters, especially ethyl acetate and iso-amyl acetate, that are typically associated with sweet or tropical flavours. These decreases were observed over time for the averages between all treatments and although the transposed data did not show significant differences between the treatments, there was some anecdotal evidence showing slightly higher retention of these acetate esters in the cork control sample and lower retention for the 30´23 Compound (Liner 7).
Figure 3: Typical histogram of slip and break torque values measured throughout the trial
Figure 4: Average free SO2 content for all treatment types, excluding baseline values (A sample filled using N2 sparging)
Conclusion
The results of this study have indicated that differences do exist between the various metal closure liners that were investigated. The organoleptic results highlighted problems with the PET/EPE 300 liner and the 30´23 Compound liner (Liners 6 and 7). In most cases the wine from samples using these liners did not perform very well organoleptically, and caution should therefore be exercised when using them in future. It is important to note, however, that the wine used in this trial was not specifically earmarked for bottling under screw cap.
Although it can be accepted that winemaking practices could have influenced the individual performance of the closures, the relative response in this trial indicated a preference for one of three liners, namely the Saran TIN (2), Poly/Alu (3) and EPE Saran (5), while the performance of the Saranex liner (1) was also acceptable. Based on the evidence of this study, it is recommended that one of these four inner liners be favoured in future bottling of wine under screw cap. Considering the torque measurements of these four liners, potential difficulty for some customers to open them was highlighted with regard to Liner 5. It is possible that this problem may be corrected through adjustments in the capping process, but such considerations fell beyond the scope of this study.
As a parting thought, even though this trial showed that the selection of a liner can influence the perceived quality of a wine, it can not in and of itself make a wine great. The success of a product starts with the correct winemaking practices and careful management of the production process. After first ensuring production of a top quality wine, such a wine may then be protected by the selection of the correct closure.
Acknowledgements
I would like to acknowledge the Winetech Bottling, Packaging and Distribution Sub-Committee, laboratory personnel at KWV and Distell, MCG and the company of wine people for their valued contributions to this project.
References
Stelzer, T. (2005). Taming the screw: A manual for winemaking with screw caps. P. 305. Brisbane: Wine Press.
Vierra, T. (2003). Wineries Down Under turn to scewcaps, Practical Winery & Vineyard, September/October issue, 32 - 37.
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